Stator Compound Having an NBIR Terpolymer Elastomeric Base and Stators and Downhole Motors Using the Same

ABSTRACT

A stator compound including an acrylonitrile butadiene isoprene rubber (NBIR) terpolymer elastomeric base.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.15/414,494 filed Jan. 24, 2017, which is a divisional of U.S.application Ser. No. 14/941,240 filed Nov. 13, 2015, which claims thebenefit of U.S. Provisional Application No. 62/175,854 filed Jun. 15,2015, all of which are incorporated herein by reference for allpurposes.

FIELD OF INVENTION

The present invention relates in general to oil and gas drillingequipment, and in particular to a stator compound having NBIR terpolymerelastomeric base and stators and downhole motors using the same.

BACKGROUND OF INVENTION

In oil and gas drilling industry, the downhole motor (“mud motor”) isnormally disposed between the end of the drill string and the drill bit.When drilling mud is pumped down the drill string from the surface, themud motor rotates the drill bit allowing it to cut through thesurrounding material. Downhole motors are particularly useful indirectional drilling, which requires steering the drill bit in a desireddirection.

A typical downhole motor includes a rotating shaft (“rotor”) disposedwithin a fixed housing (“stator”). The rotor is helically-shaped and isoffset from the helically-shaped elastomeric inner lining of the statorsuch that when drilling mud is pumped down the drill string, the rotorrotates relative to the stator. The rotor in turn drives a transmission,which converts the eccentric motion of the rotor into circular motion todrive the drill bit.

The elastomeric inner stator liner must not only withstand themechanical stresses placed upon it by the rotation of the rotor, butmust also resist deterioration from exposure to the components of thedrilling mud. However, even though mud motors have been used in the oiland gas drilling industry for many years, the ability to formulate ahigh-performance stator compound for fabricating the stator lining hasproven to be a significant challenge, particularly for stator liningsused in high-stress environments, such as those encountered in shale oildrilling operations,

Previous attempts have been made to use NBR copolymers in statorcompounds; however, the resulting stator liners provided poor dynamicperformance, were subject to high swell in diesel-based drilling muds,and had lower tear and flex fatigue resistance at elevated temperatures(e.g., 200-350° F.).

SUMMARY OF INVENTION

The principles of the present invention are embodied in a statorcompound and including an NBIR (acrylonitrile butadiene isoprenerubber), XNBIR (Carboxylated NBIR) or a HNBIR (Hydrogenated NBIR)terpolymer elastomer base. The isoprene content is nominally in therange of 5-50%, and preferably in the range of 10-30%. The elastomer mayalso include a bound antioxidant. In one preferred embodiment, theMooney viscosity range is 30 to 85, the acrylonitrile content range is28-50%, and, for embodiments using HNBIR, the residual unsaturation isin the range 0.5-20.0%.

The principles of the present invention are also embodied in a processfor making a high-performance stator compound with an NBIR terpolymerelastomeric base. The process includes at least two mixing steps, thefirst for dispersing high reinforcing carbon black and other additivesinto the NBIR terpolymer elastomer to optimize flow and high temperaturemechanical properties, and the second for adding curing agents.Preferably, in the first mixing step, carbon black is added to the NBIRelastomer and then mixed at less than about 300° F. for 1-5 minutes toachieve optimum dispersion. All of the other ingredients, except for thecuring agents, are then added and the batch is dumped and cooled toambient temperature. In the second step, the curing agents are mixedinto the batch at a temperature of less than about 225° F.

In the preferred embodiment, a high reinforcing carbon black is used inthe elastomer with the following properties: (1) Surface area of 80-250(Iodine # mg/g); and (2); Structure of 70-178 (DOP Absorption-cc/100 g).In preferred embodiment, semi-reinforcing carbon black is also includedin the elastomer, having: (1) Surface Area of 20-50 (Iodine # mg/g); and(2) Structure of 60-125 (DOP Absorption-cc/100 g). The use ofsulfur-accelerator, peroxide and phenolic cure systems are preferablyused in making the NBIR stator compound.

Stator compounds according to these principles are particularly suitablefor fabricating elastomeric stator liners for use in both downholemotors and downhole pumps, although not limited thereto. In particular,the use of a NBIR terpolymer in a stator compound has been shown toproduce improved swell properties in diesel-based drilling muds andenhanced dynamic mechanical properties such as modulus, tensilestrength, flex-fatigue life and tear resistance at high temperatures(200-350° F.). Stators molded from the inventive stator compound haveshown superior performance in downhole drilling motors, including longerrun times under heavy loads with no “chunking” of the stator lining.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1A is a perspective view of a downhole motor assembly suitable fordescribing a typical application of the principles of the presentinvention;

FIG. 1B is a top cross-sectional view of the rotor and stator assemblyof FIG. 1A; and

FIG. 2 is cutaway view showing a portion of the stator of FIGS. 1A and1B including a molded stator elastomeric liner fabricated using an NBIRterpolymer according to the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The principles of the present invention and their advantages are bestunderstood by referring to the illustrated embodiment depicted in FIGS.1-2 of the drawings, in which like numbers designate like parts.

FIG. 1A is a diagram of an exemplary downhole motor assembly 100, whichattaches to the end of a drilling string during oil and gas drillingoperations. Exemplary downhole motor assembly 100 includes a top sub101, stator and rotor assembly 102, transmission 103, offset oradjustable housing 104, bearing assembly 105, and drill bit 106. FIG. 1Bis an end cut-away view of stator and rotor assembly 102 and shows thehelical rotor 109 offset within the helical liner 108 of stator tube107.

FIG. 2 is a side cutaway view of stator and rotor assembly 102 of FIG.1, which shows stator tube 107 and associated molded stator elastomerlining 106 in further detail. The fabrication of molded stator elastomerlining 108 is one exemplary use of a NBIR terpolymer embodying theprinciples of the present invention.

Moreover, stator compounds of according to the principles of the presentinvention are also suitable for the fabrication of NBIR terpolymerstator liners used in downhole pumps. (Generally, downhole motors andpumps are structurally similar, and both implement power conversion,with downhole motors typically used to provide torque and rotation tothe drill bit and downhole pumps typically used to provide pressure andflow rate for moving fluids. The typical downhole pump stator will haveas few as two lobes and the typical downhole motor stator will havebetween three and ten lobes.)

Several elastomers types and cure systems were investigated by theinventors. The focus was to fabricate a stator lining with good dynamicmechanical properties, low swell in diesel-based muds and good hotmechanical properties at 200-350° F. It was found that the use of NBIRterpolymer in stator compounds improved dynamic mechanical propertiessuch as sealing, hysteresis (Tan Delta) modulus, tensile strength, tearand flex-fatigue resistance. An isoprene unit was selected to be thesame as in natural rubber, which is known to have these good dynamicproperties. The isoprene content is nominally in the range of 5-50%, andpreferably in the range of 10-30%. The acrylonitrile group (ACN)provides the resistance to drilling fluids such as oil and diesel-basedmuds.

According to one preferred process, the selected NBIR Terpolymer ismixed in a production rubber mixer using a two step mixing procedure toensure dispersion of high reinforcing carbon black and other additives.In addition, this procedure also produces optimum flow (viscosity)properties and mechanical strength. A sulfur, sulfur phenolic orperoxide/co-agent cure system is preferably used to cure the compound.This cure system was developed to produce optimum scorch and curingproperties. A blend of plasticizers is used to achieve good flow anddiesel extraction properties.

Table 1 illustrates the preferred ingredients for the first step inproducing NBIR stator compounds according to the principles of thepresent invention. Ranges for the preferred ingredients are listed inparts per one hundred parts of rubber (PHR). In particular, the highreinforcing carbon black has a surface area of 80-250 (Iodine # mg/g)and structure of 70-178 (DOP Absorption-cc/100 g) and thesemi-reinforcing carbon black has a surface area of 20-50 (Iodine #mg/g) and structure of 60-125 (DOP Absorption-cc/100 g).

During the first step, the ingredients in Table 1 are mixed in acommercial rubber mixer for approximately 4-10 minutes and a temperaturebelow 300° F. The mixture (i.e., the first pass masterbatch) is thenallowed to cool to ambient temperature prior to the second mixing step.

TABLE 1 INGREDIENTS (1^(ST) MIX CYCLE) PHR NBIR TERPOLYMER 100.0ANTIOXIDANT 0.2-2.0 ANTIOZONANT 0.5-3.0 SEMI-REINFORCING CARBON BLACK 40-120 HIGH REINFORCING CARBON BLACK 5.0-40  POLYMERIC PLASTICIZER 5.0-40.0 ESTER PLASTICIZER  5.0-20.0 PHENOLIC RESIN 10.0-50.0 ZINCOXIDE 3.0-5.0 STEARIC ACID 1.0-5.0 HEXAMETHYLENE TETRAAMINE 1.0-8.0RETARDERS 0.2-4.0

The preferred ingredients for the second mixing step are shown in Table2. During the second mixing step, the ingredients of Table 2 are mixedfor approximately 2-5 minutes at less than 225° F., after which mixtureis dumped from the mixer and onto a two-roll mill.

TABLE 2 INGREDIENTS (2^(ND) MIX CYCLE) 1^(ST) PASS MASTERBATCH PHRSULFUR 0.50-5.0 RUBBER ACCELERATORS  0.5-5.0

The resulting NBIR terpolymer compound has been shown to have goodresistance in water and diesel-based drilling fluids. This compound alsoshows good dynamic mechanical properties, flex fatigue life and tearresistance at high temperatures (200-350° F.). The high value at lowstrain modulus (i.e., less than 25%) reduces flexing of the stator lobe,which results in improved performance.

The characteristics of the preferred embodiment of the present NBIRterpolymer stator compound are provided in Table 3:

TABLE 3 RHEOLOGICAL, CURE AND MECHANICAL PROPERTIES FOR NBIR STATORCOMPOUND VISCOSITY MOONEY VISC.  20-100 (ASTM D1646) (ML1 + 4) @ 100° C.MU MOONEY SCORCH >60 T5@100° C. minutes RHEOLOGY MDR (Rheometer) 60′ @160° C. (ASTM 2084) ML (dNm)¹ 0.5-3.0 MH (dNm) 20-80 TS (2) minutes1.5-5.0 TC (90) minutes  5.0-25.0 Tan Delta 0.10-0.30 MICRODUMBELL HOTPROPERTIES (ASTM D1708) TENSILE STRENGTH (psi)²  800-2000 Tested @ 121°C. ELONGATION % 125-300 25% MODULUS (psi) 250-600 TEAR DIE C @ 121° C.(ppi)  50-200 (ASTM D624) TROUSER @ 121° C. (ppi)³  5-100 DIN ABRASION(ASTM D5963) cured 30′@177° C. 100-250 (mm³) DeMattia Flex Final CrackWidth (mm)   18.5 (ASTM D813) Cycles to Final Width  5,000-20,000Initial piercing is 2 mm wide per ASTM D813. Width of specimen is 25.4mm. Capillary Load: 35 kg Shear Rate (s⁻¹)  5-20 Rheometer Kg—kilograms(ASTM D5099) Viscosity (Pa · s)⁴ 100-400 Die: 1.0 mm × 2.0 mm/Temp: 100°C. DIESEL-BASED MUD RESISTANCE 72 H @ 250° F. % VOLUME CHANGE -+5.0/−2.0 SHORE A HARDNESS CHANGE +5/−5 25% MODULUS (% CHANGE) +25/−10TENSILE STRENGTH (% CHANGE) +25/−10 ELONGATION (% CHANGE) +10/−25 ¹dNm =decinewtons meter ²Psi = pounds per square inch ³Ppi = pounds per inch⁴Pa · s = Pascal · second

Although the invention has been described with reference to specificembodiments, these descriptions are not meant to be construed in alimiting sense. Various modifications of the disclosed embodiments, aswell as alternative embodiments of the invention, will become apparentto persons skilled in the art upon reference to the description of theinvention. It should be appreciated by those skilled in the art that theconception and the specific embodiment disclosed might be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

It is therefore contemplated that the claims will cover any suchmodifications or embodiments that fall within the true scope of theinvention.

1-10. (canceled)
 11. A method of making a stator compound, comprising:during a first mixing stage, mixing a high reinforcing carbon black withan elastomeric base at a temperature below about 300 degrees Fahrenheitwithout curing agents to produce a mixture, wherein the elastomeric basehas one or more rubber components, wherein the only rubber components ofthe elastomeric base are acrylonitrile butadiene isoprene rubbers(NBIR); and during a second mixing stage, mixing curing agents into themixture.
 12. The method of claim 11, wherein mixing a high reinforcingcarbon black with the elastomeric base comprises mixing a highreinforcing carbon black having a surface area of about 80 to about 250(Iodine # mg/g) and a structure of about 70 to about 178 (DOPAbsorption-cc/100 g) with the elastomeric base.
 13. The method of claim11, further comprising mixing a semi-reinforcing carbon black with theelastomeric base during the first mixing stage.
 14. The method of claim13, wherein mixing a semi-reinforcing carbon black with the elastomericbase comprises mixing a semi-reinforcing carbon black having a surfacearea of about 20 to about 50 (Iodine # mg/g) and a structure of about 60to about 125 (DOP Absorption-cc/100 g) with the elastomeric base. 15.The method of claim 11, further comprising mixing at least oneplasticizer selected from the group consisting of polymeric and esterplasticizers with the elastomeric base during the first mixing stage.16. The method of claim 11, wherein mixing curing agents into themixture comprises mixing sulfur into the mixture during the secondmixing stage.
 17. The method of claim 11, further comprising mixing anantioxidant with the elastomeric base during the first mixing stage. 18.The method of claim 11, further comprising mixing at least one of aphenolic resin without a curing agent, zinc oxide, stearic acid, andhexamethylene tetraamine with the elastomeric base during the firstmixing stage.
 19. A stator for use in a downhole motor or pump andhaving a stator liner, the stator liner formed of a stator compoundhaving a elastomeric base, the elastomeric base having one or morerubber components, wherein the only rubber components of the elastomericbase are acrylonitrile butadiene isoprene rubbers (NBIR).
 20. The statorof claim 19, wherein the rubber components are selected from the groupconsisting of hydrogenated acrylonitrile butadiene isoprene rubber(HNBIR), acrylonitrile butadiene rubber that has not been hydrogenated,and carboxylated acrylonitrile butadiene isoprene rubber (XNBIR). 21.The stator of claim 19, wherein the stator liner is formed of a statorcompound further including a least one of a high reinforcing carbonblack and a semi-reinforcing carbon black.
 22. The stator of claim 19,wherein the stator liner is formed of a stator compound furtherincluding a high reinforcing carbon black having a surface area of about80 to about 250 (Iodine # mg/g) and a structure of about 70 to about 178(DOP Absorption-cc/100 g).
 23. The stator of claim 19, wherein thestator liner is formed of a stator compound further including asemi-reinforcing carbon black has a surface area of about 20 to about 50(Iodine # mg/g) and a structure of about 60 to about 125 (DOPAbsorption-cc/100 g).
 24. The stator of claim 19, wherein the statorliner is formed of a stator compound further including at least oneplasticizer selected from the group consisting of polymeric and esterplasticizers.
 25. The stator of claim 19, wherein the stator liner isformed of a stator compound further including an antioxidant.